I uploaded a video with my problem. I want to make this truck drivable. How can I fix this problem?
Do the tires have physics constraint? It seems it requires those and then lock momentum on 1 axis
if you want use standart vehicle class, create skelet(in your 3d app and skin wheels and body meshe to that skelet) with minimum five joints(root, and for all wheels), all joints MUST be in x front, y left, z UP direction axis(in your video you have wrong axis setup)!!!
i agree with the others eg bone alignment . on top of that the root bone MUST be at 0,0,0